This invention relates generally to high temperature gradient casting apparatus utilising a molten metal crystallizer and more particularly to floating heat insulating baffles for shielding the cooling bath from high temperature radiating from such casting furnace, and to a method of directional casting of articles using such ceramic baffles.
The casting of single crystal and directionally solidified superalloys requires the cooling of the superalloy in a temperature gradient. Typically, the temperature gradient is achieved by melting the superalloy in a high temperature furnace and then slowly withdrawing the superalloy from the furnace and lowering it into a liquid metal cooling bath, commonly referred to a crystallizer bath, which causes corresponding gradual solidification of the cast article. The best superalloy mechanical properties are achieved when the highest temperature gradients are used. Common metals for use as a crystallizer bath include tin and aluminium.
A typical apparatus for casting directionally solidified metals is illustrated in U.S. Pat. No. 4,108,236 (Salkeld). Salkeld shows an apparatus having a high temperature furnace suspended over a molten metal bath. Beneath the heating chamber is a floating insulating baffle. Openings through the bottom of the furnace enable a mold to be lowered therethrough.
Materials used for floating baffles must be chemically stable with respect to the liquid crystallizer, have low thermal conductivity, and a density low enough (and a displaced volume high enough) to allow the material to float on the liquid crystallizer. The most stable ceramic materials with respect to liquid aluminum crystallizers are alumina (aluminum oxide) and zirconia (zirconium oxide). Different methods of lowering the density of alumina, zirconia or other chemically compatible materials have been used. For example, the desired material has been formed into integral hollow ceramic bubbles. Unfortunately, the method of manufacturing substantially spherical, hollow ceramic bubbles produces a very thin wall that is easily broken. This makes the substantially spherical bubbles unsuitable as a floating baffle material because whenever a bubble is broken, the ceramic will sink to the bottom of the crystallizer vessel. Bubbles with a thicker, stronger wall are not currently available and a manufacturing technique to make a thicker, stronger wall on the bubbles is not known.
The Salkeld reference discussed above teaches a discshaped, heat insulating baffle constructed of a fibrous zirconia insulating core bonded in a sandwich-type arrangement between heat resistant graphite sheets as being suitable for applications in molten tin at 500xc2x0 F. (250xc2x0 C.).
Published EPO 0631832A1 illustrates a floating thermal insulating layer 13 which floats on a cooling bath used in a directional solidification casting process (see FIG. 4 thereof). It is disclosed that the thermal layer xe2x80x9cconsists of a material capable of flowingxe2x80x9d and that the mold when lowered into the cooling bath xe2x80x9cpenetratesxe2x80x9d the thermal layer. The material of the insulating layer is disclosed to be prepared from granules of graphite, ceramic, or aluminum oxide with a coating that prevents wetting, such as boron nitrate. Alternately, solids of boron nitrate or spherules of Si AlO2N are disclosed as being capable of being used. Undesirably, however, as mentioned above, where aluminum is used as the molten metal cooling bath, dense solids (including boron nitrate) will typically be too dense to float on such cooling bath. In addition, integral hollow spherules of SiAlO2N, are generally unsuitable since the method of manufacture produces, as mentioned above, a thin wall that is easily broken or cracked with a resultant of loss of insulating capability.
Each of the above references provides that the insulating baffle disclosed in such references possess an opening for lowering the mold into the cooling bath. This configuration has the drawback that there exists an uninsulated portion of the cooling bath through which conduction of heat can readily occur.
Likewise, Russian patent No. 1401715 teaches a floating thermal baffle 5 for a cooling bath used in a directional casting furnace. Such baffle 5 is constructed of alternating layers of heat resistant material 6 (graphite). Again, due to the provision of an opening in such baffle 5 to allow for the lowering of the cast metal and mold into the cooling bath, such thermal layer and casting method suffers from the same disadvantages of Salkeld and EPO 0631832A1, namely an opening in the floating thermal baffle which results in undesirable convection of heat, thus undesirably reducing the thermal gradient between the cooling bath and the mold being lowered into such bath.
In order to overcome the aforesaid disadvantages of the prior art, the present invention in one broad aspect thereof provides a method for providing a floating thermal insulating layer over a liquid metal cooling bath used in a directional casting furnace, which avoids having to provide an opening in the thermal layer. A plurality of hollow ceramic baffles are provided, which float in the surface of the crystallizer cooling bath. Such plurality of baffle members provide an insulating layer and advantageously are displaced only to the extent caused by the lowering of the mold into the crystallizer during the directional solidification process. Due to their floating and displaceable characteristics, the individual baffle members surround and conform to the exterior periphery of the mold when it is lowered into the crystallizer bath, advantageously maintaining the thermal insulating layer as close to the mold as possible.
Moreover, upon removal of the mold and cast article from the cooling bath, the floating baffles, due to their displaceable nature, then float to their original position substantially re-covering all of the surface of the cooling bath.
In order to ensure floatation of such baffles in cooling mediums such as molten aluminum, the ceramic members of the present invention are each constructed in a particular manner to create a hollow hermetically sealed chamber therein. Accordingly, in one of the broadest aspects of the present invention, a method of providing an insulative heat barrier over a surface of a cooling bath during directional casting of superalloys is provided, comprising the steps of substantially covering said surface with a plurality of ceramic baffle members, such ceramic baffle members each comprising a hollow ceramic member and ceramic seal means sintered to said hollow member so as to form a hermetically sealed chamber within said hollow member, such baffle members floatable on the surface of the cooling bath and substantially non-reactive with the cooling bath.
The ceramic baffle members are each assembled from the hollow ceramic member and seal means when each are in a green (unsintered) state, and subsequently both sintered to each other to form the sealed chamber. It is provided, in a preferred embodiment, that the ceramic seal means be situate within the hollow member and adapted to shrink less than the hollow member during the sintering process, so that the hollow member is xe2x80x9cshrink-fitxe2x80x9d around the seal means so as to assist in the seal means hermetically sealing the chamber of the baffle member.
It is preferred that the ceramic baffle member be formed as an extruded ceramic tube (of any shape such as cylindrical, rectangular, square, or triangular), and having a pair of respectively opposite distal ends, and that the seal means comprise a pair of diepressed thin flat end members adapted for respective insertion in the distal ends of the ceramic tube member. When the end members are inserted in a green state into the opposite distal ends of the hollow ceramic member (also in a green state), and the assembly fired, the end members become sintered to the hollow member creating a hermetically sealed chamber sealed at both ends by the respective end members. Where the end members are of a ceramic material which shrinks less than the ceramic material of which the hollow members are comprised, this feature will assist the hollow member in being shrink fit around the periphery of the end members and assist the end members in creating a hermetic seal at the respective ends of the hollow tube member. In the preferred embodiment the ceramic tube member is substantially cylindrical, and the end members comprise thin, substantially flat circular disks.
The preferred cylindrical configuration for the baffle members possesses a number of advantages. Firstly, it is relatively easy to form an extruded ceramic tube member in a hollow cylindrical configuration, which may be cut in sections to form hollow members for a number of ceramic baffles. Secondly, where the density and enclosed volume of a cylindrical baffle member is such that a longitudinal axis of the cylindrical tube member lies parallel to, but above, the surface of the molten cooling bath, subsequent or unintended solidification of the cooling bath will not result in compression of the ceramic baffles within the metal bath and possible breakage of the baffle due to compressive forces exerted thereon during the cooling or heating the now-solidified metal. Instead, during any expansion of the molten metal arising from solidification or heating, due to the rounded nature of the cylindrical tube member, in the baffle member will be forced upward from the bath, and the bath will not be able to exert a compressive force on the baffle.
Advantageously, the creation of a hermetically sealed chamber within the ceramic baffle used in the method of the present invention increases the insulative properties of the baffle. Moreover, if the sintering step used during the forming of the ceramic baffles is carried out in a vacuum or partial vacuum, the vacuum so created in the sealed chamber of the baffle member will further increase the insulative properties of the ceramic baffle. The manner of construction of the ceramic baffles of the present invention is particularly suited to forming a vacuum within the sealed chamber.
Advantageously, the present invention further provides for ceramic baffles which may be constructed with relatively thick walls and having a hollow sealed chamber under a vacuum, the latter greatly adding to the insulative qualities of the baffle members. An opacifier may further be added to the ceramic material used for the hollow member and the seal means, as a means of reducing radiant heat loss through such baffles.
The hollow tube member and seal means may be comprised of similar or different ceramic material, and preferably of ceramics of different composition or at least of different thermal expansion, provided that each ceramic material is capable of being sintered to the other. In the preferred embodiment the hollow tube member and the seal means are comprised of ceramics of identical composition, but of different green densities, so that the assembly, when heat is applied during the sintering process, will, due to the different green densities and different rates of expansion/contraction, cause the tube member and the seal means to be press-fit together to thereby assist in creating a hermetic seal between such two components. Such hermetic seal allows the creation of a sealed inner chamber within the baffle member to thereby allow the insulative properties of the baffle member and further ensure buoyancy of the ceramic member within most liquid metal baths. As previously mentioned, to assist the seal means, typically flat disks, in plugging the distal ends of the hollow tube member, it is desired that the end disks be adapted be adapted, due to having a different green density or different co-efficient of thermal expansion, to shrink less than the hollow member which they are inserted within. Such accordingly permits the hollow member to be xe2x80x9cshrink fitxe2x80x9d about the periphery of the sealing disks, thus better ensuring a hermetic seal is created by the end members and a sealed chamber is created within the baffle member. This ensures no molten metal will be permitted to leak into the sealed chamber, thus advantageously maintaining the insulative properties of the baffle member. Typical satisfactory ceramic materials for the hollow member and the seal means include alumina (aluminum oxide), silicon carbide, silicon dioxide, mullite, and zirconia (zirconium oxide), although other ceramics and ceramics/compounds may be selected and the invention is not limited to the aforesaid ceramics and ceramics/compounds.
In a further broad aspect of the present invention, a method of producing single crystal and directionally solidified superalloys is provided, in which:
a) said superalloy is melted in a high temperature furnace;
b) said molten superalloy is transferred into a ceramic mold; and
c) said ceramic mold and contained molten superalloy are lowered into a molten metal cooling bath, comprising the steps of:
i) floating a plurality of ceramic baffle members on a surface of said cooling bath to substantially cover said surface to form an insulating layer for reducing heat transfer from said furnace into said cooling bath via said surface, said ceramic baffles being substantially non-reactive with said molten metal cooling bath; and
ii) lowering said mold through said insulating layer of ceramic baffle members into said cooling bath.
In another particular aspect of the present invention, a ceramic baffle is provided, adapted for floating on a surface of a cooling bath, for use in a directional casting furnace, a plurality of which adapted to form a floating insulative heat barrier over the surface of said cooling bath, comprising:
a hollow ceramic member;
ceramic seal means sintered to said hollow member so as to form a hermetically sealed chamber within said hollow member.